1. Field of the Invention
The present invention relates generally to separating low frequency and high frequency components, and more specifically to analysis and feedback from an electrode or probe located in a plating bath.
2. Background of the Invention
Plating is a method of putting an outer coating on the surface of an underlying metal. Popular uses of plating include anodizing the outer surface of aluminum and galvanized steel, and plating copper onto electronic circuit boards. In general, the above plating processes do not require ultra-accurate plating equipment, and can generally accommodate and withstand impurities which may be resident in the plating bath at the beginning of the plating process, or residual impurities, commonly referred to as degradation products.
In many plating applications, however, the characteristics of the plated surface are extremely critical, frequently being more important than the characteristics of the underlying material in some cases. The characteristics of plated coatings usually help determine tensile strength, solderability, brightness, ductility, or flexibility, uniformity of coating, and resistance to thermal or electrical shock. These characteristics of plated coatings are highly dependent on the composition of the plating bath from which they are deposited on an underlying substrate.
Historical platings of copper and the like were made by inserting an electrode made of soft, pure copper into an acid bath, and passing a current through the electrode and the acid bath in order to deposit copper on a circuit board, or the like. The copper that was deposited was usually viewed as a dead, soft grade copper, whose thickness was determined by the current flowing through the electrode, and the amount of time the circuit board spent in the bath. As newer plating surfaces are desired, it has become necessary to apply small amounts of trace elements, compounds, as well as mixtures of plating materials, all in pre-determined quantities, in order to obtain the characteristics of the plating surface required.
This has forced a more analytical evaluation of the composition of the plating bath, both before and during the plating process in order to ensure that the proper mixture of materials is present, and is plated in the desired manner on the base material. U.S. Pat. No. 4,132,605 describes one of the methods which has been used to evaluate the composition of a plating solution. The method involves the application of a DC voltage to a working electrode which is positioned in the plating solution. The voltage is gradually varied in order to pass through both the metal plating voltage range and the metal stripping voltage range. A counter electrode is also placed in the plating solution to aid in the measurements.
This method provides a signal which is related to the plating rate and is useful in evaluating the presence and concentration of elements or compounds that assist or impede the plating process. Unfortunately, not all of the materials which are present in a plating bath will affect the plating rate. Some of the materials will significantly affect the properties of the plated layer, but will not measurably alter the plating or stripping rate. Additional problems are created when a plating bath contains several different elements or compounds which affect the properties of the plated deposit. This is especially difficult when the concentration or presence of one such material affects the way another such material is deposited.
Additional difficulties arise during the aging of the plating solution or plating bath. Aging the plating solution results in the accumulation of trace amounts of impurities and degradation products which interact with each other and material which may be added to the plating bath. It is frequently difficult to determine the precise interactions between the various constituents in the plating bath, and the effect they have on each other, as well as the effect they would have on additional material added to the plating solution.
U.S. Pat. No. 4,631,116 , assigned to the present assignee, describes a method for performing a type of spectral analysis on the contents of a plating solution. This method involves applying a gradually varying DC signal to a working electrode located in the plating bath solution. A constant frequency AC signal is superimposed on the DC voltage level. As the DC voltage level is slowly varied, the AC current of the applied AC signal is measured between the working electrode and a counter electrode which is also positioned within the plating solution. The DC voltage level is varied over a pre-determined range which includes voltages which will result in plating and stripping of the plated deposit.
The measurement of the AC current in relation to the varying DC voltage is interpreted as an AC current spectra. The DC voltage level and sweep rate, as well as the AC frequency, are optimized for each of the components within the plating bath in order to properly characterize the contents of the plating bath solution. Analysis of the resulting current spectra is made to develop a fingerprint of the composition of the plating bath. Comparison of the fingerprint of a bath having a known, desirable composition which has resulted in a desirable plating coating with the fingerprint of a plating bath of unknown composition will reveal to an operator the alterations which are required in the unknown plating bath in order to match the composition of the known plating bath, and generate a desired plated surface.
The apparatus used in the application of the method of U S. Pat. No. 4,631,116 includes a reference electrode which is essentially a standard calomel electrode, a working electrode, and a counter-electrode. The counter electrode is the source of the DC voltage and the AC signal superimposed on the DC voltage. The reference electrode or standard calomel electrode provides a reference point from which the voltage applied to the counter electrode is varied.
The method described in U.S. Pat. No. 4,631,116 requires measuring the AC current between the working electrode and a counter electrode and monitoring both the phase angle and quadriture angle of the AC current in order to determine the current spectra. The results of the measurements are recorded on a graphic display and then analyzed or compared with other, known spectra. In employing this method, it is preferable to monitor the second harmonic, which has been found to be more useful than the first harmonic in providing a detailed spectra representative of the contents of the plating bath.